Injection molding is a known discontinuous molding method used for plastic materials. Injection molding allows the production of a large quantity of molded parts, with high accuracy, which can immediately be used industrially. For this purpose, the material and/or molding mass is plasticized in an injection unit of an injection molding machine and injected into an injection molding tool. Modern injection molding machines work with a screw conveyer which plasticizes, conveys and injects the molding mass into the tool. The polymeric mass solidifies in the cavity and/or mold cavity of the tool, whereupon the molded part can be removed from the tool. A volume contraction resulting from solidification can basically—however, only to a certain extent—be compensated by a holding pressure applied prior to removal of the part.
Certain thermoplastics, duroplastics and elastomers can be processed by means of injection molding. Using masses containing fibers in injection molding thermoplastics has also been known, whereby molded parts with comparatively higher strength can be produced. During injection molding, the mass temperature is approx. between 100° C. and 350° C., the injection pressure approx. between 400 bar and 1600 bar and the tool temperature approx. between 40° C. and 160° C. When processing thermoplastics, the conveyor screw is relatively warm in order to melt the thermoplastic material. In contrast, the tool is relatively cold in order to cool the just molded part. When processing duroplastics, the temperature of the plasticizing unit has to be adjusted to the flow and cure behavior and is therefore in general lower than for thermoplastics, while the tool has to be kept warmer than the injection unit so that the mass can adequately harden there.
In contrast to calandering, by means of which only plane layers of uniform thickness can normally be produced, in injection molding, the hollow space of the tool, the so-called cavity, determines the shape and superficial structure of the finished part, so that molded parts of the type mentioned above—with rotationally symmetrical or in particular at least partially annular cross section—can be produced in a single operation by injection molding. For this purpose, the plasticized polymeric mass is injected into the cavity through an injection opening, whereby the mass flows in an annular manner around an inner core of the tool, and merges at the side opposite the injection opening. A weld line results at this point, which normally has a lower strength than the remaining body of the molded part.
It is the object of the present invention to provide a method of the type mentioned above, by means of which a molded part with a rotationally symmetrical, in particular, annular, or at least partially annular cross section, such as a nut, a collar, a pipe connector, a pipe coupling or similar part, can be produced with improved strength in the stress direction and, preferably, also controlled shrinkage and warping during solidification of the polymeric mass using simple technology.